I think you should reissue certificates but you may want to consider doing a cost benefit analysis first. Reissuing certificates could increase interest both positive and negative from members who may have become 'sleepers'. You could reissue the certificates as part of a marketing campaign. I would also check what your legal obligations are particularly if your members signed a membership agreement, you may find that you are required to let them know of the name change in which case, a new certificate might also be useful for PR. You might just decide to reissue certificates to those members who actively participate in your organisation's services as a thank you for their interest. If member's renew their interest annually, you could wait until their membership is renewed before you decide to give them a new certificate.